The present invention relates to a method for forming a breathable film, and more particularly, to a method for forming a breathable film comprising a blend of a thermoplastic polymer with an inorganic material. The breathable film is particularly useful as a backsheet on a disposable absorbent article.
Infants and other incontinent individuals wear disposable absorbent articles such as diapers to receive and contain urine and other body exudates. Absorbent articles function both to contain the discharged materials and to isolate these materials from the body of the wearer and from the wearer""s garments and bed clothing. Disposable absorbent articles having many different basic designs are known to the art. It is also known that the exterior of disposable diapers can be covered with a flexible, liquid and vapor impervious sheet to prevent any absorbed liquid from passing through the diaper and soiling adjacent articles such as clothing, bedding and the like. These outer covers, generally referred to as backsheets, are often constructed from fluid impervious films such as polyethylene. Although such backsheets do prevent liquid from passing through the diaper, they also can make the diaper feel hot and uncomfortable to wear because of their impermeability to air and/or moisture.
Backsheets which are pervious to vapor are generally known as breathable backsheets and have been described in the art. In general, these backsheets are intended to allow the passage of vapor through them while retarding the passage of liquid. For example, U.S. Pat. No. 3,156,242 issued to Crowe, Jr. on Nov. 10, 1964 teaches the use of a microporous film as a breathable backsheet. U.S. Pat. No. 3,881,489 issued to Hartwell on May 6, 1975 teaches a breathable backsheet comprising in combination two layers, the first of which is a low void volume perforated thermoplastic film and the, second of which is a porous high void volume hydrophobic tissue. U.S. Pat. No. 3,989,867 issued to Sisson on Nov. 2, 1976 teaches a breathable backsheet provided with tapered hollow bosses which prevent the passage of liquids while allowing vapors to readily pass.
Of the breathable backsheets, microporous films are most commonly used. Microporous films typically comprise a blend of a thermoplastic polymer and an inorganic filler such as calcium carbonate. The blend undergoes pore formation upon stretching as the inorganic filler separates from the polymer due to stress concentration. The formation of micropores permits the film to be breathable allowing the passage of vapor through the micropores while retarding the passage of liquid.
It is an object of the present invention to provide a novel process for forming a breathable film from a blend of a thermoplastic polymer with an inorganic material.
In accordance with the present invention, there is provided a method for forming a breathable film. The film is comprised of a blend of a thermoplastic. polymer with an inorganic material being selected from the group consisting of calcium carbonate, clay and titanium dioxide, with the preferred inorganic filler being calcium carbonate. The blended film is fed between a pair of opposing first and second rolls. The first roll comprises a plurality of toothed regions spaced apart by a plurality of grooved regions wherein the toothed regions comprises a plurality of teeth. The second roll comprises a plurality of teeth which mesh with the teeth on the first roll. As the film is fed between the first and said second rolls the portion of the film passing between the grooved regions on the first roll and the teeth on the second roll remains substantially planar forming a first region in the film while the portion of the film passing between the teeth on the first roll and the teeth on the second roll is formed into second regions having raised rib-like elements.